1. Field of the Invention
The present invention relates to a method of evaluating a semiconductor device and an apparatus for performing the same, and more particularly, to a method and apparatus for evaluating the reliability or lifetime of a semiconductor device by analyzing weak luminescence emitted from the device.
2. Description of the Prior Art
With the miniaturization of semiconductor devices, it has been a major reliability problem that device characteristics are degraded when hot carriers generated by a high electric field created in an operating device are injected and caught in the gate oxide film thereof.
Heretofore, the lifetime of a semiconductor device considering degradation of characteristics due to hot carriers has been measured by actually applying a stress voltage to the semiconductor device. However, this measurement method requires a considerable period of time and a substantial number of samples. Furthermore, the application of the method is limited to the evaluation of individual transistors.
In recent years, there has been developed apparatuses which detect and visualize weak luminescence caused by hot carriers. In such an apparatus, the positions and amounts of luminescence within an integrated device such as an LSI are detected to evaluate the lifetime characteristics of the device affected by degradation of device characteristics. Examples of such an apparatus are Hot Electron Analyzer C3230 of Hamamatsu Photonics (Hamamatsu, Japan), and Insulating Film Destruction Detector EMMI of KLA Instrument Corporation (Calif., U.S.A.).
FIG. 11 illustrates the former apparatus. When evaluating the lifetime of a semiconductor device 3 by the apparatus of FIG. 11, the device 3 is mounted in a microscope 2 placed in a darkroom 1, and illuminated by a light source 4. The image of the device 3 enlarged by the microscope 2 is input to a TV camera 6 via a photomultiplier 5, and then displayed on a display 7. A controller 8 controls these units and stores this image from the TV camera 6 in its internal memory. Next, with shielding the semiconductor device 3 from light, a voltage of a predetermined level is applied to the device 3 from a power source 9. Weak luminescence emitted from the semiconductor device 3 is magnified through the microscope 2, and is then input to the photomultiplier 5. The intensity of the luminescence is multiplied to a visible level so that the luminescence image is picked up by the camera 6 to be displayed on the display 7, while the amount or intensity of the luminescence is accumulated as the form of the number of photons and stored in the internal memory of the controller 8 for a predetermined period of time. By superimposing the image obtained as a result of accumulating the luminescence over the previously stored image of the device 3 in the controller 8, it is possible to detect and measure the positions and distribution of the luminescence. Then, it is evaluated that a device with a greater amount of luminescence has a greater number of hot carriers generated, and therefore has a shorter lifetime because the device characteristics are degraded by hot carriers.
However, the above-mentioned technique has drawbacks as described below. FIG. 12 shows the gate voltage dependence of the amount of luminescence measured by the conventional apparatus shown in FIG. 11 with regard to an N-channel MOSFET in which a prefixed drain voltage of 7 V is applied. FIG. 12 also shows the gate voltage dependence of a characteristic degradation rate .DELTA. gm/gm after application of a voltage stress for 10.sup.3 sec which is obtained under the same condition. The results shown in FIG. 12 indicate that devices with greater amounts of light emission do not necessarily have a shorter lifetime due to the degradation of device characteristics, and that the peak of luminescence amount deviates from that of the device characteristic degradation. This means that it is not possible to accurately estimate the device life by evaluating the luminescence amount alone. Another problem with the conventional equipment is that, unless the wavelength distribution of the luminescence amount is taken into account, the measured luminescence amount has a value inherent in the system because corrections are not made with respect to factors such as the spectral sensitivity of the apparatus and the absorption of light as it passes through lenses.